JP-A-60-25653 discloses a grinding apparatus wherein a load current of an electric motor for rotating a grinding wheel is measured so as to determine a point of time at which the grinding wheel is to be dressed. The grinding resistance increases as the grinding wheel becomes dull, resulting in an increased load applied to the grinding wheel and the consequently increased amount of heat generation. The increased load applied to the grinding wheel or the increased amount of heat generation causes changes in the hardness, residual stress and other mechanical properties of a workpiece subjected to the grinding operation. In this view, where the grinding wheel has become dull to a certain extent, the grinding wheel is dressed so as to restore its original sharpness. However, in the grinding apparatus disclosed in JP-A-60-25653 in which the point of time of the dressing operation is determined on the basis of change of the measured load current of the electric motor for rotating the grinding wheel, the determined dressing time does not always coincide with a dressing time suitable for the workpiece.
On the other hand, if the hardness and residual stress of the workpiece are directly measured, it would be possible to determine the dressing time suitable for the workpiece, on the basis of the measured hardness and residual stress. However, the hardness and the residual stress of the workpiece cannot be measured while the grinding operation is in process. The workpiece has to be removed from the machining apparatus, so that the hardness and the residual stress are measured by measurement devices which are independent of the machining apparatus. Since the workpiece has to be removed from the machining apparatus, the working efficiency is problematically reduced. Further, although the hardness and the residual stress of each workpiece are measured simultaneously with the grinding operation for the next workpiece, the measurement of the residual stress generally requires a long time, resulting in a delay in the point of time of the dressing operation and a risk of producing some defective products.
Such a problem arises also in a cutting operation. When the cutting tool becomes dull, the surface smoothness of the workpiece is deteriorated. Further, the cutting resistance increases thereby resulting in increased elastic deformation of the workpiece and the cutting tool, or increased thermal expansion of the workpiece and the cutting tool due to increased amount of heat generation. Either one of the increased elastic deformation and the increased thermal expansion causes deterioration in the machining accuracy. In view of this, it is preferable that the cutting tool be re-ground at a time suitable for the workpiece, so that the cutting tool restores its original sharpness at the time suitable for the workpiece.
The above-described problems are important particularly in a machining process with cold-gas-blow cooling. In the machining process with cold-gas-blow cooling, a blow of cold gas (a stream of a gas having a low temperature) is used instead of a cutting or grinding liquid or other liquid coolant, so as to be applied to a machining point at which the workpiece is machined by the machining tool, thereby cooling the machining point, as described in, for example, JP-A-56-9166. This machining process has recently begun to be practiced in view of freedom from splashes of the liquid coolant, easier recycling of the cutting chips, and other advantages thereof. However, since the gas has a lower thermal conductivity and a smaller thermal capacity than those of the liquid, the machining tool and the workpiece tend to be insufficiently cooled in this machining process. Thus, the increased amount of heat generation derived from the deterioration of the sharpness of the machining tool is likely to affect the mechanical properties of the workpiece and the machining accuracy. Therefore, it is particularly important that the dressing of the grinding wheel, the re-grinding of the cutting tool, or other rectification of the machining tool be executed at the time suitable for the workpiece.